Thread Context Restoration in a Multithreading Computer System

1. A computer system, comprising: a configuration comprising a core configurable between a single thread (ST) mode and a multithreading (MT) mode, the ST mode addressing a primary thread and the MT mode addressing the primary thread and one or more secondary threads on shared resources of the core; and a multithreading facility configured to control utilization of the configuration to perform a method comprising: based on switching from the MT mode to the ST mode, disabling the one or more secondary threads, wherein a thread context comprising program accessible register values and program counter values of the one or more secondary threads is made unavailable to programs; querying a last-specified maximum MT level in the ST mode to determine a last-set program-specified maximum thread-id of the configuration; and based on the last-set program-specified maximum thread-id indicating MT, obtaining the thread context of the one or more secondary threads by performing a) and b) comprising: a) executing a set MT instruction to resume the MT mode; and b) based on being in the resumed MT mode, accessing the thread context of the one or more secondary threads.

2. The computer system according to claim 1 , wherein the set MT instruction is a signal processor instruction comprising a set MT order and a program-specified maximum thread-id indicating MT.

3. The computer system according to claim 1 , further comprising saving the thread context of the one or more secondary threads based on switching to the ST mode.

4. The computer system according to claim 1 , wherein the switching from the MT mode to the ST mode is based on a non-clearing reset operation, and further wherein the executing of the set MT instruction to resume the MT mode and the accessing of the thread context of the one or more secondary threads is performed by a stand-alone dump program.

5. The computer system according to claim 4 , wherein the stand-alone dump program specifies the last program-specified maximum thread-id as a program-specified maximum thread-id when issuing a set-MT order to resume the MT mode.

6. The computer system according to claim 1 , wherein a clearing reset is performed prior to loading an operating system that does not support MT in the configuration to prevent an MT-aware stand-alone dump program from attempting to dump any secondary threads from the configuration.

7. The computer system according to claim 1 , wherein a program that is MT-aware but does not exploit MT, issues a set-MT order with a corresponding maximum thread-id of zero prior to executing a stand-alone dump program for the configuration.

8. The computer system according to claim 1 , wherein the last program-specified maximum thread-id is retained until a clear reset or a deactivation of the configuration occurs.

9. A computer program product for thread context restoration in a configuration comprising a core configurable between a single thread (ST) mode and a multithreading (MT) mode, the ST mode addressing a primary thread and the MT mode addressing the primary thread and one or more secondary threads on shared resources of the core, the computer program product comprising: a computer readable storage medium having program instructions embodied therewith, wherein the computer readable storage medium is not a signal, the program instructions readable by a processing circuit to cause the processing circuit to perform a method comprising: based on switching from the MT mode to the ST mode, disabling the one or more secondary threads, wherein a thread context comprising program accessible register values and program counter values of the one or more secondary threads is made unavailable to programs; querying a last-specified maximum MT level in the ST mode to determine a last-set program-specified maximum thread-id of the configuration; and based on the last-set program-specified maximum thread-id indicating MT, obtaining the thread context of the one or more secondary threads by performing a) and b) comprising: a) executing a set MT instruction to resume the MT mode; and b) based on being in the resumed MT mode, accessing the thread context of the one or more secondary threads.

10. The computer program product according to claim 9 , wherein the set MT instruction is a signal processor instruction comprising a set MT order and a program-specified maximum thread-id indicating MT.

11. The computer program product according to claim 9 , further comprising saving the thread context of the one or more secondary threads based on switching to the ST mode.

12. The computer program product according to claim 9 , wherein the switching from the MT mode to the ST mode is based on a non-clearing reset operation, and further wherein the executing of the set MT instruction to resume the MT mode and the accessing of the thread context of the one or more secondary threads is performed by a stand-alone dump program.

13. The computer program product according to claim 9 , wherein the last program-specified maximum thread-id is retained until a clear reset or a deactivation of the configuration occurs.